1. Field of the Invention
The present invention relates to a method of coating fluid-penetrable articles with a polymeric disperson, preferably a water-dispersible polymeric coating, and includes the steps of pre-heating a fluid-penetrable article, such as a compactible gasket; thereafter coating the fluid-penetrable article with a polymeric disperson; conveying the coated article through a drying station on a plurality of support members, such as continuous elastomeric cords to allow the coated article to dry to a tack-free condition; and then conveying the coated article through a curing oven for curing the deposited coating material.
Many fluid-permeable compactible articles, such as gaskets, are coated with a polymeric dispersion, for example, a polymeric dispersion, such as chloroprene synthetic rubber blended with phenol formaldehyde resin or nitrile synthetic rubber blended with phenol formaldehyde resin. Articles, such as gaskets, with these type coatings, and after curing, provide fluid-impermeability during use. For example, an automotive transmission oil pan gasket requires a relatively thick uniform coating of a soft conformable material for sealing, principally due to the fact that the clamping flanges are relatively lightweight metal with rather wide spacings between the fasteners. The flanges are not only quite flexible, but in substantially all cases quite uneven. The clamp load on these type applications is very unveven--relatively high under the fasteners and relatively low between the fasteners, because the flanges bend readily between fasteners. The coating material provides the fluid barrier needed in the areas of the part that are lightly clamp loaded. The coatings selected are of course tailored to resist the environmental conditions such as the particular fluid, the temperatures, and the torque loading entailed with each application. The gasket material to be coated likewise is selected to provide the resistance to the environmental conditions in the particular application. In the case of a pan gasket, these gasket materials are commonly vegetable fibers blended with granulated cork and reinforced with elastomeric materials such as chloroprene rubber or nitrile rubber and in some cases SBR synthetic rubber. Frequently, mineral fibers are used--blended with vegetable fibers.
2. Prior Art
A variety of gasket materials have been coated with polymeric dispersions for many years. My prior U.S. Pat. No. 3,770,480 discloses this assignee's coating method used immediately prior to this invention for applying polymeric coatings to gasket materials. In general, a die cut gasket is coated on both major surfaces and its edges with a blend of synthetic elastomers and phenolic resin in a water or solvent dispersion. When in the form of a die cut part, the gasket includes one or more pilot holes so that it can be hung on a hook from the pilot hole for drying and then curing of the polymeric coating. In this manner the polymeric coating is not disturbed during drying or curing so that the coating material will maintain its fluid-impermeability. In this prior art method, the gaskets coated with a wet, tacky polymer are manually hung onto hooks from the pilot holes and are sent on a long, continuous path by a chain conveyor to provide sufficient time for the applied coating to be partially dryed prior to curing. The polymeric coating on the hung gaskets is dryed while the gaskets are hanging from the pilot hole until the polymeric coating is substantially tack-free. The dryed, coated gaskets are then conveyed through a curing station, while hung from the pilot holes, and since the major coated surfaces of the gaskets do not contact the conveyor, the impermeability of the coating is not disturbed during drying or curing. The above-noted drying step prior to curing is required for a very important reason--to permit a major portion of the fluid from the polymeric coating, which has partially penetrated the compactible material, to escape before the coating is subjected to the relatively high temperature curing (for example 325.degree.-375.degree. F.). The curing gasifies the fluid (for example water) during the curing step and, at the same time, seals the polymeric surface to render the coating impermeable. Any significant amount of fluid that has not escaped during the drying step becomes locked into the part, and will cause the polymeric coating to blister or `balloon` outwards.
Drying by the above-described hook-hanging method is quite slow and requires very careful monitoring of the room or chamber conditions, particularly temperature and humidity, where the drying is accomplished. The drying operation can be speeded up using higher temperatures upwards to approximately 110.degree.-130.degree. F. but has to be very carefully controlled to be sure that the surface of the coating does not seal or "skin over" by premature curing to lock in fluid from the polymeric coating and thereby slow down the drying step rather than speed it up. Accordingly, drying must be carried out slowly at a temperature well below the curing temperature of the polymeric coating, generally room temperature. In actual practice, air drying at room temperatures in the range of 65.degree.-100.degree. F., which requires a range of 20-80 minutes, has been found to be safest and is typical for the above-described hook hanging drying procedure. Drying these type of coatings sometimes has to be stopped when the room humidity becomes too high until there is a weather change. Three or four warm days with, for example, 85.degree. F. with 90% humidity, will cause a shutdown of the drying operation handling the above-described coated gasket materials.
The time and temperatures required for drying by the hook-hanging method are affected by the particular porosity of the many varieties of compactible gasket materials used. A gasket material will pick-up or absorb fluid from the applied polymeric coating in proportion to its porosity. It has been necessary to subject coated, die-cut gaskets to a substantial air-dry period in the range of about 20-80 minutes at 65.degree.-100.degree. F., after coating and before curing, to make sure that the water picked up by the gasket material from the polymeric coating has been driven off before the coating "skins over" on the surface. The coating materials are designed to be fluid-impermeable and if these coatings "skin over" or surface dry before the gasket-obtained water migrates out from the gasket sheet and through the polymeric coating, the retained water or solvent(s) will vaporize at the high temperatures in the curing stage causing blistering of the gasket.
Considerable skill has been developed in the air-drying/hook-hanging method so that blistering can be controlled by assuring that the hook-hung gaksets travel through a lengthy air drying path. Depending on the temperature and humidity conditions in the room in which the hook-hung parts are dried, sometimes warm, dry air is circulated to reduce the time necessary to drive off the water picked up by the gasket material. This air circulation technique must be controlled very carefully, however, since the coating material can be caused to "skin over" too quickly before the moisture from the gasket material has migrated out from the sheet as described above. In any event, each part must be individually hung on hooks of the conveyor while the conveyor moves through a large room at a temperature of 65.degree.-100.degree. F. and at varying relative humidity during the year.
In my prior U.S. Pat. No. 3,986,915, there is disclosed a method of fabricating a valve plate for transmissions including a metal core having a gasket material adhered to one or both core surfaces. The gasket material can be a polymeric material and the metal core can be preheated for cover sheet adherence and to reduce subsequent curing time of the adhesive. The gasket material, however, is applied to the metal core as solid sheet material in U.S. Pat. No. 3,986,915 and, since the metal core material is fluid-impermeable, fluid penetration into the core material is not a problem.
As set forth in my U.S. Pat. No. 3,926,445, prior techniques for coating gasket materials with liquid dispersions of polymers include coating one side of the gasket material with a very thin coating of the polymeric dispersion so that the coating material can dry readily. The thin coating technique sometimes requires several coating applications. The present invention relates to a method of applying a liquid dispersion of a polymer to a fluid-permeable core material in such a manner that the fluid-permeable core material is not substantially permeated with liquid from the applied coating. In this manner, the coating can be applied uniformly in any desired thickness while achieving rapid drying of the polymeric coating without causing coating surface defects.